Link between the Nankai underthrust turbidites and shallow slow earthquakes

Trench sediments such as pelagic clay or terrigenous turbidites have long been invoked to explain the seismogenic behavior of the megathrust fault (i.e., décollement). Recent numerous studies suggest that slow earthquakes may be associated with huge megathrust earthquake; however, controls on the slow earthquake occurrence remain poorly understood. We investigate seismic reflection data along the Nankai Trough subduction zone to understand the correlations between the spatial distribution of the broad turbidites and along-strike variations in shallow slow earthquakes and slip-deficit rates. This report presents a unique map of regional distribution of the three discrete Miocene turbidites that underthrust apparently along the décollement beneath the Nankai accretionary prism. A comparison of distributions of the Nankai underthrust turbidites, shallow slow earthquakes, and slip-deficit rates enables us to infer that the underthrust turbidites may cause primarily low pore-fluid overpressures and high effective vertical stresses across the décollement, leading to potentially inhibiting the slow earthquake occurrence. Our findings provide a new insight into potential role of the underthrust turbidites for shallow slow earthquakes at subduction zone.

www.nature.com/scientificreports/ PSP, is estimated to have opened between 25 and 15 Ma by backarc spreading of the Izu-Bonin arc 15 . The > 100 km wide Nankai accretionary prism, which has developed landward of the trench since the Miocene, mainly consists of offscraped and underplated materials from the trough-fill turbidites and the Shikoku Basin sediments 16 . Incoming sediments to subduction zones get involved in not only the growth and recycling of continental crust but also megathrust and/or slow earthquake occurrence through the accretion process [17][18][19][20][21][22][23] . Trench sediments such as pelagic clay or terrigenous turbidite have long been invoked to explain the seismogenic behavior of the megathrust fault, for example, the Middle America Trench 24 , the Sunda Trench 25 , and the Japan Trench 26 . A numerical modeling study suggested that lithostratigraphy of the underthrust sediment can control dewatering and pore-fluid pressure in the footwall of the western Nankai, where thick turbidite sequence subducts 27 . Although many geological and geophysical investigations 11,[28][29][30][31] have identified the turbidites, few studies have obtained a complete view of the turbidite distribution for the entire Nankai subduction zone, except for the Tilley et al. 32 where the turbidite distribution is largely confined to regions seaward of the trench axis. Moreover, a possible link between the underthrust turbidites and shallow slow earthquakes remains less well documented for the entire Nankai subduction zone. In this paper we show several multi-channel seismic (MCS) reflection profiles that reveal incoming turbidites facies to the Nankai subduction zone, and document along-strike variations in the underthrust sediments, Figure 1. Seafloor topography, physiographic features, MCS lines, turbidite distribution, slow earthquake activity, and décollement properties in the Nankai Trough margin offshore southwest Japan. (a) Inset: Regional map showing the location of the study area (red box). Gray thin lines mark the locations of the MCS survey lines. Blue thick parts on MCS lines mark the MCS profiles shown in Figs. 2 and 3. Thick purple, green, and orange parts on dip and strike MCS lines mark western turbidite (WT), central turbidite (CT), and eastern turbidite (ET) facies within the Shikoku Basin sedimentary section, respectively. Slip-deficit rate (SDR) distributions 1 are shaded in red. Dark cyan dots mark shallow slow earthquakes of very low frequency earthquakes 5-8 and low-frequency tremors 9 www.nature.com/scientificreports/ décollement properties, and slow earthquakes. Moreover, we present potential implications of the underthrust turbidites for the shallow slow earthquakes and fault strength of the décollement.

Results
We performed lithostratigraphic interpretation on the MCS profiles (Fig. 1a  www.nature.com/scientificreports/ sediment, because laterally continuous sandstones may act as effective drainages for the dewatering. Pickering et al. 31 named this unit Kyushu Fan. Hereinafter, we call the unit A as 'western turbidite' (WT). We observe ~ 120-km-wide unit B offshore Cape Shiono (CS) of the Kii Peninsula on the MCS profile of line NT0501H. Unit B is composed of four discrete channel bodies that are lens-shaped or filling basement lows. This unit shows relatively continuous and partially chaotic reflections. Underwood and Pickering 34 interpreted unit B to be equivalent to Kyushu Fan (turbidites), even though its geologic age and lithology are unknown due to lack of drilling through this unit. In order to distinguish it from unit A, we call the unit B as 'central turbidite' (CT).
We identify two different units C and D offshore Kumano of the east Kii Peninsula, which are ~ 100 km wide. Units C and D are characterized by discontinuous reflections that have a higher amplitude than the surrounding strata. Drilling at IODP Site C0011 recovered ~ 140-m-thick volcanic turbidite with late Miocene age for unit C (Zenisu Fan; upper eastern turbidite), ~ 195-m-thick hemipelagic mudstone, and ~ 176-m-thick mixed (siliciclastic and volcaniclastic) turbidite with middle Miocene age for unit D (Kyushu Fan; lower eastern turbidite) from top to bottom 31 . In fact, the Kyushu Fan at Site C0011 may be thicker than the ~ 176 m, because the drilling stopped within the turbidite section so that the sediment-basement interface was not cored. Hereinafter, we call an entire layer from units C down to D as 'eastern turbidite' (ET), which includes the interbedded hemipelagic mudstone facies. In summary, Fig. 1a presents a unique map of regional distribution of the three discrete turbidites (WT, CT, and ET) for the entire Nankai Trough margin, which is compiled by the MCS and drilling data. We observe the underthrust turbidites over ~ 420 km (ca. 78%) on a ~ 540-km-long profile east-west along the Nankai Trough.

Discussion
Based on seismic reflection characteristics and ocean drilling results, we mapped the three discrete turbidites (WT, CT, and ET) with Miocene age for the entire Nankai subduction zone. As earlier studies on the Nankai Trough turbidites, Ike et al. 30 documented regional and local variations in basement relief, sediment thickness, and sediment type in the Shikoku Basin. They focused on turbidite sedimentation in the Shikoku Basin on incoming Philippine Sea Plate, which reflects basement control on deposition, leading to the local presence or absence of turbidite units. Tilley et al. 32 investigated the seismic reflection characteristics of incoming sediments to know sediment and basement variations related to the along-strike changes within the accretionary prism and plate boundary conditions at the Nankai Trough. In this paper we spatially mapped the turbidites and expanded their distribution landward from the Nankai Trough to investigate their control on seismogenic behavior of the décollement with integration of the turbidite subduction, slip-deficit rates, slow earthquakes, the décollement reflection polarities, and surface slope angles of the accretionary wedge.
Potential role of the underthrust turbidites for slow earthquakes. Comparing spatial distribution of the underthrusting turbidites to slow earthquakes (Fig. 1a), we found that forearc regions offshore CA and CS, where the underthrusting WT and CT are expected to extend landward respectively, are largely consistent with the low activity of slow earthquakes (Fig. 1b). These regions are almost characterized by high SDRs indicative of high interplate coupling conditions, which were determined by seafloor geodetic observations 1 . In contrast, many slow earthquakes have been observed in a forearc region offshore CM which monotonous mudstones with low permeability subduct beneath the accretionary prism, without the underthrusting turbidites. A numerical simulation study 27 on dewatering and pore-fluid pressure of subducting sedimentary sequence demonstrated that sandstone-dominated system with enhanced drainage has lower pore-fluid overpressures and higher effective stresses than mudstone-dominated system, leading to lower porosity and greater stiffness. The numerical study also suggests that the less-permeable mudstone layer could be poorly drained and likely to be much overpressured, leading to low effective stresses in the footwall beneath the décollement. Assuming that the shallow slow earthquakes are closely associated with high pore-fluid pressure 8 as well as material heterogeneity, geometric complexity and deformation at low differential stress 35 , we infer that the Nankai underthrust turbidites containing permeable sandstones may cause primarily low pore-fluid overpressures and high effective vertical stresses across the décollement, leading to high interplate coupling and thus potentially inhibiting the shallow slow earthquakes. This inference is consistent with simulated pore-fluid pressures 36  www.nature.com/scientificreports/ www.nature.com/scientificreports/ distance offshore CM. Elevated pore pressures reduce the effective vertical stress and thus lower the shear stress necessary for failure of the décollement.

Along-strike variations in the underthrust sediments, décollement properties, and slow earthquakes. Reflection polarities (reverse or normal) of the décollement suggest states of physical
properties along the décollement and beneath it 11 , which may influence pore-fluid pressures of the décollement and therefore slow earthquakes. The reverse polarity reflection (e.g., Fig. 3a,d) is caused by a decrease in acoustic impedance (velocity × density) across the décollement as an interface between a high-impedance hanging wall (accretionary prism) and a low-impedance footwall (underthrust sediment). The reverse-polarity reflection simply suggests that the décollement and underthrust sediment are poorly drained and highly overpressured. The normal polarity reflection (e.g., Fig. 3b,c) is caused by an increase in acoustic impedance across the décollement between a low-impedance hanging wall and a high-impedance footwall. A previous study by Park et al. 11 revealed a variation in the décollement reflection polarities along the Nankai subduction zone; however, controls on the variation remained unclear. Here we propose that the décollement reflection polarities are more likely related to variations in the turbidite subduction, pore-fluid overpressures, and slow earthquakes along the Nankai shallow subduction zone, as below. Offshore Hyuga of the Kyushu and CM of the Shikoku Island, many shallow slow earthquakes have been observed (Fig. 1a). It is likely related to high pore-fluid overpressures and thus reverse-polarity reflection (Fig. 1b) of the décollement, consistent with lack of underthrust turbidites. Offshore CA of the Shikoku Island, we unexpectedly observed the reverse-polarity reflection of the décollement, despite a correlation of three phenomena (underthrust turbidites, potentially low pore-fluid overpressures, and few slow earthquakes). A possible scenario to explain this is that the décollement offshore CA might be somewhat overpressured despite the underthrusting WT so that it can exhibit the reverse-polarity reflection, but its overpressure is not actually that high enough to facilitate the slow earthquake activity.
Offshore CS of the Kii Peninsula, we observed alterations of normal and reverse polarity reflections of the décollement. Consistent with low activity of the shallow slow earthquake, the normal-polarity reflection inherently suggestive of low pore-fluid overpressure may be attributed to the underthrust CT. In contrast, the reverse-polarity reflection could be caused by the underthrust CT deposited within isolated basement lows and erosional channels (Figs. 2b and 3b) that may generate local compartments of overpressure in the footwall, because the turbidites pinch out against the flanks of surrounding topographic highs and thus their pore-fluid escape could be inhibited 32,37,38 . Nevertheless, the overpressure appears not to be that high enough to cause many slow earthquakes.
Control of the underthrust turbidites on the décollement reflection polarity and shallow slow earthquakes appears more complicated offshore Kumano of the east Kii Peninsula where we observe alterations of normal and reverse polarity reflections of the décollement. Because the décollement occurs within the underthrust upper ET (i.e., unit C on MCS line KR0108-4, see Fig. 3c) with possibly enhanced drainage, we can simply expect low pore-fluid overpressures and high effective stresses developing in the footwall, leading to low activity of slow earthquakes and the normal-polarity reflection of the décollement. As a matter of fact, however, many shallow slow earthquakes have been observed offshore Kumano. Moreover, reflections of reverse polarity as well as normal polarity are observed along the décollement. A possible explanation for this unexpected case is that the underthrust ET containing the massive less-permeable mudstone layer (e.g., ~ 195 m thick at Site C0011) may cause relatively high pore pressures and low effective stresses across the décollement, favoring the reverse-polarity reflection and slow earthquakes. Moreover, the channel deposits 31 of the upper ET would help to maintain the overpressured décollement. Alternatively, such frequent shallow slow earthquakes offshore Kumano may be associated with: (1) a subducting ridge 12 potentially creating a complicated fracture network and thus generating slow earthquakes; (2) the presence of a low seismic velocity zone 16 suggesting high fluid pressures 39 and thus favoring slow earthquakes; (3) two large intra-slab earthquakes (M7.2 and 7.5) with thrust-type focal mechanism that occurred offshore Kumano in 2004, which may have induced the pore-fluid migration along the pre-existing faults or created a new fracture network, thereby triggering the slow earthquakes 5 .

Décollement fault strength estimation by a simple topographic parameter. Wedge taper angles
(surface slope α plus basal dip β) are largely controlled by the frictional strength of the décollement 40 . A recent theoretical study on critical taper model demonstrated that seafloor surface slope angle could be a first-order approximation to account for effective coefficient of basal friction for convergent margin wedge and thus the strength of décollement when the pore-fluid pressure ratio (λ) is high, internal friction (φ) is small, or both 41 . Koge et al. 41 calculated WOA (weight of alpha (α)) of 70% based on (λ, φ) = (0.7, 27°), and therefore suggested that their method can be applicable to the Nankai outer accretionary wedge. We accepted their suggestion because the Nankai internal friction (φ = 27°) is much small although the pore-fluid pressure ratio (λ = 0.7) is not that high, compared to other 21 subduction zones with average values (λ, φ) = (0.88, 34°) for which the theoretical study was done. In fact, the strength or pore-fluid pressure conditions in the footwall of the décollement do not directly govern the critical taper model, because the frictional strength of the décollement is affected by frictional coefficient which is a product of intrinsic (or material) friction coefficient and pore-fluid pressure. To simplify the argument over the variation in basal friction strength across the Nankai margin, we assume that physical properties in the footwall primarily affect frictional strength of the décollement. This assumption is consistent with the numerical study 27 suggesting that sediments in the underthrust sequence may hold implications for the overlying décollement, as they mediate the mechanical strength and elastic strain accumulation in the footwall. Average surface slope angles (α = 3.3 to 3.8°) of the Nankai outer ~ 40-km-wide accretionary wedge offshore CA and CS, characterized by subduction of the WT and CT respectively, are higher than that offshore CM (α = 1.6°) www.nature.com/scientificreports/ (Fig. 4), suggesting the stronger shear strength of décollement offshore CA and CS. This supports our inference that the underthrust turbidites and expected low pore-fluid overpressure and high effective stress across the décollement probably play an important role in facilitating the interplate coupling and eventually reducing shallow slow earthquakes offshore CA and CS. In contrast, the décollements offshore Hyuga, CM, and Kumano are weakly coupled and prone to slip. Interestingly, the average surface slope angle (α = 3.3°) offshore Kumano is relatively high and very similar to that offshore CS (Fig. 4), even though the surface angle is expected to be lower because of potentially overpressured décollement. We speculate that both a subduction of basement high below the frontal prism (~ 5 to 15 km distance in Fig. 3c) and steep splay fault in the outer ridge 42 might help growth of the surface slope angle. Tilley et al. 32 argued that the wedge taper may be more correlative (correlation coefficient ρ = 0.61) with the thickness of the mud-dominant facies than the turbidite thickness, referring to much low correlation (ρ = 0.17) between the turbidite thickness and wedge taper angle. When we focus on the underthrust sediment facies (permeable sand-dominant (i.e., turbidite) or less permeable mud-dominant) potentially affecting the wedge taper, their results are in contrast with not only our inference above but also previous studies 27,29,36 that the mud-dominant sedimentary layer would maintain high pore pressures and low wedge tapers. The low correlation (ρ = 0.17) between the turbidite thickness and wedge taper angle could be underestimated probably because their interpretation on the underthrust turbidites offshore CS and Kumano was almost limited to the incoming sequence to the Nankai Trough, which is much improved in our study. A recent preliminary estimation of porefluid pressures using seismic interval velocities (i.e., velocity-porosity-effective stress transformations) along the Nankai décollement 43 showed a general trend of higher pore pressures for the décollement of mudstonedominated system offshore CM than that of sandstone-dominated system offshore CS, still supporting our inference above and the previous studies.
Summary and future perspectives. We investigate seismic reflection data along the Nankai Trough subduction zone to understand the correlations between the spatial distribution of the three broad underthrust turbidites and along-strike variations in shallow slow earthquakes (very low frequency earthquakes (VLFEs) and low frequency tremors) and slip-deficit rates. We find that while the first two turbidites seem to correlate with a lack of VLFE activity and high slip-deficit rates along strike, the third easternmost turbidite is more complicated and correlate with the VLFE occurrence. We speculate that the underthrust turbidites with potentially permeable sandstones may cause low pore-fluid overpressures and high effective vertical stresses across the décollement, leading to inhibiting the slow earthquake occurrence. However, the connection between the underthrust turbidite and the fault slip mechanics that may produce VLFEs along the décollement is not established yet. Moreover, pore-fluid pressure of the décollement is not the only factor that can lead to slow earthquakes, although high pore-fluid pressure has been hypothesized to be a key factor in slow earthquake occurrence at subduction zones. As a future study, quantitative and seamless comparison of the in-situ pore pressures over the broader Nankai décollement would be required to validate our inference. Laboratory experiments to test the frictional behavior of the Nankai décollement samples recovered in the sandstone-dominated (i.e., turbidite) and mudstone-dominated systems would be useful for understanding on fault slip behavior associated with the shallow slow earthquakes.

Methods
Seismic reflection data acquisition and processing. The MCS data presented in this paper (Fig. 1a) were acquired by R/V Kairei of the Japan Marine Science and Technology Center (JAMSTEC) from 1997 through 2010. For deep-penetration seismic imaging, a large volume (~ 200 L) air gun array was used as the controlled sound source, except for a trench-parallel line of NT0501H on which two GI guns (~ 12 L) were used to obtain high resolution images of the incoming sedimentary layer to the Nankai Trough. The MCS data were recorded with a 4000 m, 160-channel streamer with 25 m group spacing, except for the line of NT0501H with a 5100 m, 204-channel streamer. Conventional MCS data processing techniques were applied to all the data, including trace editing, pre-filtering, spherical divergence correction, signature deconvolution, common midpoint (CMP)